System and method for superposition coding and interference cancellation in a mimo system

ABSTRACT

The present disclosure relates generally to a system and method for superposition coding and interference cancellation in a multiple input multiple output (MIMO) system. In one example, the method includes demultiplexing a signal of a user into at least first and second signal portions and demultiplexing a signal of another user into at least third and fourth signal portions. Superposition coding is performed on the first and third signal portions to form a first composite signal and performed on the second and fourth signal portions to form a second composite signal. The first composite signal is transmitted via at least one antenna of a MIMO system and the second composite signal is transmitted via at least another antenna of the MIMO system.

CLAIM OF PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/814,439, filed on Jun. 16, 2006, which isincorporated by reference herein in its entirety.

This application is related to U.S. patent application Ser. No.11/554,686, filed on Oct. 31, 2006, and entitled “WIRELESS COMMUNICATIONMETHOD AND SYSTEM FOR COMMUNICATING VIA MULTIPLE INFORMATION STREAMS”,U.S. patent application Ser. No. 11/554,726, filed on Oct. 31, 2006, andentitled “WIRELESS COMMUNICATION SYSTEM AND METHODOLOGY FORCOMMUNICATING VIA MULTIPLE INFORMATION STREAMS”, and U.S. patentapplication Ser. No. ______ (Attorney Docket No.2007.02.002.WS0/1005.38), filed on Apr. 23, 2007, and entitled “SYSTEMAND METHOD FOR BROADCAST PRE-CODING IN A MIMO SYSTEM”, which areincorporated by reference herein in their entirety.

BACKGROUND

In a wireless system, different methods may be used to maximize thesignal capacity that may be transmitted. However, current methodscontain inefficiencies that negatively impact system capacity andperformance, which results in inefficient use of the radio spectrum.Accordingly, it is desirable that such inefficiencies be addressed.

SUMMARY

In one embodiment, a method comprises demultiplexing a first signal of afirst user into at least first and second signal portions anddemultiplexing a second signal of a second user into at least third andfourth signal portions. Superposition coding is performed on the firstand third signal portions to form a first composite signal and on thesecond and fourth signal portions to form a second composite signal. Thefirst composite signal is transmitted via at least a first antenna of amultiple input multiple output (MIMO) system and the second compositesignal is transmitted via at least a second antenna of the MIMO system.

In another embodiment, a method comprises demultiplexing a first signalof a first user into at least first and second signal portions, anddemultiplexing a second signal of a second user into at least third andfourth signal portions. Pre-coding is performed on the first and secondsignal portions to form first and second pre-coded signals that eachcontains a portion of the first and second signal portions. Pre-codingis performed on the third and fourth signal portions to form third andfourth pre-coded signals that each contains a portion of the third andfourth signal portions. Superposition coding is performed on the firstand third pre-coded signals to form a first composite signal and on thesecond and fourth pre-coded signals portions to form a second compositesignal. The first composite signal is transmitted via at least a firstantenna of a multiple input multiple output (MIMO) system and the secondcomposite signal is transmitted via at least a second antenna of theMIMO system.

In still another embodiment, a method comprises performing superpositioncoding on a first signal of a first user and a second signal of a seconduser to create a composite signal. The composite signal is demultiplexedinto first and second composite signal portions, wherein each of thefirst and second composite signal portions contains a portion of thefirst signal and the second signal. The first composite signal portionis transmitted via a first antenna of a multiple input multiple output(MIMO) system and the second composite signal portion is transmitted viaa second antenna of the MIMO system.

In yet another embodiment, a method comprises receiving, by a receiver,at least first and second associated multiple input multiple output(MIMO) signals, wherein each of the first and second signals contains atleast corresponding first and second superimposed layers. The firstlayer of the first and second signals is decoded, and the second layerof the first and second signals after the first layer is decoded.

In another embodiment, a method comprises demultiplexing a unicastsignal into at least first and second unicast signal portions.Superposition coding is performed on the first unicast signal portionand a broadcast signal to form a first composite signal and performed onthe second unicast signal portion and the broadcast signal to form asecond composite signal. The first composite signal is transmitted viaat least a first antenna of a multiple input multiple output (MIMO)system and the second composite signal is transmitted via at least asecond antenna of the MIMO system.

In yet another embodiment, a multiple input multiple output (MIMO)system comprises a modulation block, a serial to parallel conversionblock, a superposition coding block, and first and second antennas. Themodulation block is configured to modulate a first signal into at leastfirst and second modulated symbols and to modulate a second signal intoat least third and fourth modulated symbols. The serial to parallelconversion block is configured to parallelize the first and secondmodulated symbols and the third and fourth modulated symbols. Thesuperposition coding block is configured to superimpose the first andthird modulated symbols to form a first composite signal and tosuperimpose the second and fourth modulated symbols to form a secondcomposite signal. The first and second antennas are coupled to thesuperposition coding block and configured to transmit at least the firstand second composite signals, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a diagram of one embodiment of a multiple input multipleoutput (MIMO) system.

FIG. 2 is a diagram of one embodiment of a MIMO system using a singlecode word scheme.

FIG. 3 is a diagram of one embodiment of a MIMO system using a multiplecode word scheme.

FIGS. 4 a-4 c illustrate one embodiment of superposition coding.

FIG. 5 illustrates one embodiment of a MIMO system using superpositioncoding.

FIG. 6 is a flowchart illustrating one embodiment of a method forforming a superimposed multi-layer message for transmission in a MIMOsystem.

FIG. 7 is a flowchart illustrating one embodiment of a method fordecoding a superimposed multi-layer message by a MIMO receiver.

FIG. 8 illustrates one embodiment of a process by which superimposedmulti-layer data may be decoded by a MIMO receiver.

FIG. 9 is a flowchart illustrating one embodiment of a method forsorting users based on channel capacity information.

FIG. 10 is a flowchart illustrating one embodiment of a method forselecting users and associated code words.

FIG. 11 is a diagram of one embodiment of a MIMO transmitter systemusing pre-coding.

FIG. 12 is a diagram of one embodiment of a MIMO receiver system usingpre-coding.

FIG. 13 is a diagram of one embodiment of a MIMO transmitter systemusing pre-coding after superposition coding with multiple streams peruser.

FIG. 14 is a diagram of one embodiment of a MIMO transmitter systemusing pre-coding after superposition coding with multiple streams forone user and a single stream for another user.

FIG. 15 is a diagram of one embodiment of a MIMO transmitter systemusing pre-coding before superposition coding with multiple streams peruser, where a separate pre-coding block is used for each user.

FIG. 16 is a diagram of one embodiment of a MIMO transmitter systemusing pre-coding before superposition coding with multiple streams forone user and a single stream for another user, where a separatepre-coding block is used for each user.

FIG. 17 is a flowchart illustrating one embodiment of a method forsorting users for superpositioning based on MIMO channel rank.

FIG. 18 is a flowchart illustrating one embodiment of a method forsorting users for superpositioning based on MIMO channel conditionnumber.

FIGS. 19 a and 19 b are flowcharts illustrating embodiments ofsuperpositioned signal MIMO decoding based on MIMO channel rank.

FIG. 20 is a diagram of one embodiment of a MIMO system usingsuperposition coding for a multi-stream unicast signal and a singlestream broadcast signal.

FIG. 21 is a diagram of one embodiment of a MIMO system usingsuperposition coding for a multi-stream unicast signal and amulti-stream broadcast signal.

FIG. 22 illustrates one embodiment of a process by which superimposedmulti-layer unicast and broadcast data may be decoded by a MIMOreceiver.

FIG. 23 is a diagram of one embodiment of a system for pre-coding amulti-stream broadcast signal.

FIG. 24 is a diagram of one embodiment of a system for pre-coding asingle stream broadcast signal.

FIG. 25 is a flowchart illustrating one embodiment of a method forpre-coding a broadcast signal for transmission in a MIMO system.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Referring to FIG. 1, one embodiment of a multiple input multiple output(MIMO) system 100 is illustrated. The MIMO system 100 is a 4×4 systemthat uses multiple transmit antennas 102 a-102 d and multiple receiveantennas 104 a-104 d to improve the capacity and reliability of wirelesscommunication channels. Such a MIMO system may provide a linear increasein capacity with K, where K is the minimum of number of transmit (M) andreceive antennas (N) (i.e., K=min(M,N)). The simplified example providedby the 4×4 MIMO system 100 is able to separately transmit four differentdata streams (Data Streams 1-4) from the four transmit antennas 102a-102 d. The transmitted signals are received at the four receiveantennas 104 a-104 d. In the present example, spatial signal processingmay be performed by spatial processing block 106 on the received signalsin order to recover the four data streams. For example, the spatialsignal processing may include the use of a scheme such as Vertical BellLaboratories Layered Space-Time (V-BLAST), which uses the successiveinterference cancellation principle to recover the transmitted datastreams. In other embodiments, MIMO schemes may be used that performspace-time coding across the transmit antennas (e.g., Diagonal BellLaboratories Layered Space-Time (D-BLAST)) and/or beamforming schemessuch as Spatial Division Multiple Access (SDMA).

MIMO channel estimation may include estimating the channel gain andphase information for links from each of the transmit antennas 102 a-102d to each of the receive antennas 104 a-104 d. Accordingly, a channelfor an M×N MIMO system may be described as an M×N matrix H:

$H = \begin{bmatrix}a_{11} & a_{12} & \cdots & a_{1N} \\a_{21} & a_{22} & \cdots & a_{2N} \\\vdots & \vdots & \cdots & \vdots \\a_{M\; 1} & a_{M\; 2} & \cdots & a_{MN}\end{bmatrix}$

where a_(ij) represents the channel gain from transmit antenna i toreceive antenna j. In order to enable the estimations of the elements ofthe MIMO channel matrix, separate pilot signals may be transmitted fromeach of the transmit antennas. Other embodiments of MIMO systems aredescribed in detail in previously incorporated U.S. patent applicationSer. Nos. 11/554,686 and 11/554,726.

Referring to FIG. 2, one embodiment of a single-code word MIMOtransmission system 200 is illustrated. In the present example, thesystem 200 includes a cyclic redundancy check (CRC) attachment block202, a turbo/LDPC (low-density parity-check) block 204, a modulationblock 206, a de-multiplexing (demux) block 208, and two antennas 210 aand 210 b. It is understood that blocks (e.g., demux block 208) may bedivided into multiple blocks. It is also understood that the describedfunctionality of the system 200 may be implemented in hardware,software, or a combination thereof.

In the case of single-code word MIMO transmission, a single informationblock enters the system 200 and a CRC is added to the single informationblock by CRC attachment block 202. Coding and modulation (e.g.,quadrature phase shift keying (QPSK) or quadrature amplitude modulation(such as 16-QAM)) are performed on the single information block by theturbo/LPDC block 204 and modulation block 206, respectively, and thecoded and modulated symbols are then de-multiplexed by demux block 208for transmission over antennas 210 a and 210 b.

Referring to FIG. 3, one embodiment of a multiple-code word MIMOtransmission system 300 is illustrated. In the present example, thesystem 300 includes a demux block 302, CRC attachment blocks 304 a and304 b, Turbo/LDPC blocks 306 a and 306 b, modulation blocks 308 a and308 b, and two antennas 310 a and 310 b. It is understood that differentblocks having the same functionality (e.g., CRC attachment blocks 304 aand 304 b) may be implemented as different blocks or may be implementedas a single block. Furthermore, single blocks (e.g., demux block 302)may be divided into multiple blocks. It is also understood that thedescribed functionality of the system 300 may be implemented inhardware, software, or a combination thereof.

In the case of multiple-code word MIMO transmission, a singleinformation block enters the system 300 and is de-multiplexed intosmaller information blocks by demux block 302. In the present example,the single information block is de-multiplexed into two smaller blocks(denoted Stream 1 Block and Stream 2 Block), but it is understood thatthe de-multiplexing process may result in more than two smaller blocks.Individual CRCs are attached to Stream 1 Block and Stream 2 Block by CRCattachment blocks 304 a and 304 b, respectively, and then coding andmodulation may be performed on Stream 1 Block and Stream 2 Block byTurbo/LDPC blocks 306 a and 306 b, respectively, and modulation blocks308 a and 308 b, respectively. Stream 1 Block and Stream 2 Block maythen be transmitted from separate MIMO antennas (or beams) 310 a and 310b, respectively.

It should be noted that, in the case of multi-code word MIMOtransmissions, different modulation and coding may be applied to each ofthe individual streams Stream 1 Block and Stream 2 Block, resulting in aper antenna rate control (PARC) scheme. Multi-code word transmission mayallow for more efficient post-decoding interference cancellationbecause, for example, a CRC check can be performed on each of the codewords before the code word is cancelled from the overall signal.Accordingly, only correctly received code words may be cancelled,thereby avoiding any interference propagation in the cancellationprocess.

Referring to FIGS. 4 a-4 c, an example of superposition coding isillustrated. FIG. 4 a illustrates a first user (User-1) associated witha first signal (signal x1) and FIG. 4 b illustrates a second user(User-2) associated with a second signal (signal x2). As shown in FIG. 4c, signal x2 may be superimposed on signal x1, resulting in a compositesignal x that is transmitted. At the receiver, User-2 may first decodesignal x1, cancel it from the composite received signal x, and thendecode its own signal x2. User-1 may decode its own signal x1 from thecomposite received signal x without any cancellation.

Referring to FIG. 5, in one embodiment, an M×M MIMO system 500 isillustrated. The present embodiment is used to illustrate a transmissionscheme where superposition coding is performed in conjunction with eachMIMO stream transmission. User signals may be selected for positioningwithin a particular layer of a superimposed signal based on manydifferent criteria, examples of which are described in laterembodiments. Although the present example is in the context ofmulti-code word transmission, it is understood that the principlesdescribed herein may be applied to single code word MIMO schemes.Furthermore, although the present example illustrates superpositioncoding as occurring after MIMO demultiplexing, it is understood thatsuperposition coding may occur first in some embodiments. Generally, thepresent embodiment of an M×M MIMO system may enable up to SM code words(CW) to be transmitted simultaneously with M code words to each of Susers.

In the present example, the system 500 includes modulation blocks 502a-502 d (which may use any appropriate modulation type, such as QAM),serial-to-parallel (S/P) conversion blocks 504 a-504 d, gain means 506a-506 d, addition blocks 508 a-508 d, Inverse Fast Fourier Transformblocks (510 a and 510 b), parallel-to-serial (P/S) conversion blocks 512a and 512 b, and cyclic prefix (CP) addition blocks 514 a and 514 b.Although not shown, components similar to those of FIG. 3 may bepositioned prior to modulation blocks 502 a-502 d. For example, prior toeach modulation block 502 a-502 d, a demux block, CRC attachment block,and Turbo/LDPC block, as well as other blocks, may be provided asdescribed with respect to FIG. 3. It is understood that different blockshaving the same functionality (e.g., S/P conversion blocks 504 a-504 d)may be implemented as different blocks or may be the same block, andthat blocks may be further subdivided. Furthermore, it is understoodthat the functionally provided by various components of the system 500may be implemented in software, hardware, or a combination thereof.

In the multiple-code word MIMO transmission of the present example, auser's information (e.g., User-1 Data) may undergo processing such asthat described with respect to FIG. 3 (e.g., de-multiplexing, CRCattachment, and coding). This processing results in User-1 Data beingdivided into multiple data blocks, each of which is associated with anindividual code word. In the present example, User-1 Data is split intoM code words, denoted as CW D11-CW D1M in FIG. 5. Similarly, the data ofeach of the other users through the S^(th) user (User-S Data) is splitinto M code words, with User-S Data being denoted as CW DS1-CW DSM inFIG. 5.

For purposes of illustration, the progression of code words CW D11 andCW DS1 through the system 500 will now be described. CW D11 entersmodulation block 502 a and is converted to a stream of modulatedsymbols. The modulated symbols are fed into S/P block 504 a, where theyundergo a serial to parallel conversion. A power gain of g_(ij) may beapplied to the ij^(th) code word, so with respect to CW D11, a gain ofg₁₁ may be applied by gain means 506 a to each of the parallel modulatedsymbols forming CW D11. In a similar manner, CW DS1 enters modulationblock 502 b and is converted to a stream of modulated symbols. Themodulated symbols are fed into S/P block 504 b, where they undergo aserial to parallel conversion. A gain of g_(S1) may be applied to eachof the parallel modulated symbols by gain means 506 b.

The first modulated symbol of CW D11 and the first modulated symbol ofCW DS1 may then be superimposed in addition block 508 a. Each of theremaining modulated symbols may be superimposed in a similar manner,with the last modulated symbol of CW D11 and the last modulated symbolof CW DS1 being superimposed in addition block 508 b. The superimposedsymbols may then enter IFFT block 510 a in parallel, undergo IFFTprocessing, and then enter P/S block 512 a for parallel to serialconversion. A CP may then be added to the serial stream by CP additionblock 514 a prior to transmission via antenna 516 a.

Although not described in detail herein, other code words may undergosimilar superpositioning, processing, and transmission until the M^(th)code words (CW D1M and CW DSM) are superimposed, processed, andtransmitted via antenna 516 b. New data blocks for User-1 through User-Smay then be processed.

Referring to FIG. 6, in another embodiment, a method 600 illustrates aprocess by which superpositioning may occur within a MIMO system, suchas the MIMO system 500 of FIG. 5. In step 602, a first signal of a firstuser (e.g., User-1 Data of FIG. 5) may be demultiplexed into at leastfirst and second signal portions (e.g., CW D11 and CW D1M). In step 604,a second signal of a second user (e.g., User-S Data of FIG. 5) may bedemultiplexed into at least third and fourth signal portions (e.g., CWDS1 and CW DSM). In step 606, superposition coding may be performed onthe first and third signal portions to form a first composite signal andon the second and fourth signal portions to form a second compositesignal. In step 608, the first composite signal may be transmitted viaat least a first antenna (e.g., antenna 516 a) and the second compositesignal may be transmitted via at least a second antenna (e.g., antenna516 a). It is understood that the term “signal” as used in the presentexample may include code words, modulated symbols, or any otherrepresentation of data that may be processed by a MIMO system, whetherin digital or analog form.

Referring to FIG. 7, a method 700 illustrates one embodiment of adecoding process that a receiver (not shown) may use to decodesuperimposed layers, such as those encoded by the system 500 of FIG. 5.In step 702, a minimum mean square error (MMSE) operation may beperformed. In each of the following steps, M code words in a singlesuperimposed layer may be decoded. For example, M code words may bedecoded in superimposed layer 1 in step 704, M code words may be decodedin superimposed layer 2 in step 706, and M code words may be decoded insuperimposed layer S in step 708. It is understood that, if thereceiver's information is in a higher layer (e.g., layer 2), then lowerlayers (e.g., layer 3) may not be decoded.

Referring to FIG. 8, a more detailed embodiment of the method 700 ofFIG. 7 is illustrated. The present embodiment illustrates a decodingprocess 800 that a receiver in a multi-stream MIMO system may use todecode superimposed layers, such as those encoded by the system 500 ofFIG. 5. In this example, each of the superimposed layers includes amulti-stream MIMO transmission of order M (e.g., M code words).Generally, a superimposed layer k consists of code words Sk1, Sk2 . . .SkM. As described with respect to FIG. 5, each superimposed layer may beintended for a different user. Therefore, each user receives M codewords (assuming multi-code word MIMO transmission). A user correspondingto the k^(th) superimposed layer would need to decode k−1 superimposedlayers (i.e., (k−1)M code words) before decoding its own layer (i.e.,the k^(th) layer). The layer decoding example of the present figureapplies to a user associated with the last layer (i.e., the user needsto decode all the superimposed layers before reaching its own layer).However, it is understood that a user may stop layer decoding afterdecoding its own layer. For example, a user corresponding to the k^(th)superimposed layer may stop processing after decoding the k^(th)superimposed layer, even if other layers remain encoded.

For purposes of illustration, FIG. 8 continues the example of FIG. 5with the use of code words CW D11-CW D1M and CW DS1-CW DSM. In thereceiver, M code words are received at block 804 for S users as datay1-yM from antennas 802 a-802 d (e.g., ant1-antM). Although not shown,it is understood that block 804 may perform spatial signal processingand other operations to reform the signals received via antennas 802a-802 d. As such operations are commonly known, they are not describedin detail herein.

Beginning with code word D11, an MMSE operation is performed in block806 and the code word is decoded in block 808. The decoded code word D11is then cancelled from the composite signal in block 810, and theresulting signal is fed into block 812. In block 812, an MMSE operationis performed and the next code word D12 is decoded in block 814. Thedecoded code words D11 and D12 are then cancelled from the compositesignal in 816, and the resulting signal is fed into the next block. Thisprocess may continue until the last code word in superimposed layer 1 isreached, which is code word D1M. Accordingly, an MMSE operation isperformed in block 818 and code word D1M is decoded in block 820. Ifthere is another superimposed layer, the decoded code words D11-D1M arethen cancelled from the composite signal in block 822, and the resultingsignal undergoes the next level of processing. It is understood thatprocessing may end at this point if the receiver corresponds to theinformation in superimposed layer 1.

In the present example, the receiver corresponds to the lastsuperimposed layer S and processing continues after layer 1. Althoughnot shown, it is understood that many layers may be decoded betweenlayer 1 and layer S. In block 824, an MMSE operation is performed andthe code word DS1 is decoded in block 826. The decoded code wordsD11-DS1 are then cancelled from the composite signal in block 828, andthe resulting signal is fed into block 830. In block 830, an MMSEoperation is performed and the next code word DS2 is decoded in block832. The decoded code words D11-DS2 are then cancelled from thecomposite signal in block 834, and the resulting signal is fed into thenext block. This process may continue until the last code word insuperimposed layer S is reached, which is code word DSM. As code wordDSM is the last code word in the last layer, a maximum ratio combining(MRC) operation may be performed in block 836 and the last code word DSMis decoded in block 838.

Referring to FIG. 9, in another embodiment, a method 900 illustrates thesuperposition coding of user signals based on the users' MIMO channelcapacities. In step 902, users may be sorted by a system (e.g., thesystem 500 of FIG. 5) for superposition coding based on MIMO channelcapacity using MIMO channel capacity information 910. Generally, thecapacity of an M×M MIMO channel may be denoted by:

$C_{MIMO} = {{E\left\lbrack {\log_{2}\text{det}\left( {I_{M} + {\frac{SNR}{M}{HH}^{*}}} \right)} \right\rbrack}\mspace{14mu}\left\lbrack {{b/s}/{Hz}} \right\rbrack}$

where SNR is the received signal-to-noise ratio at each receive antenna.

It is noted that, in some embodiments, users may account for theirreceiver types when calculating the MIMO channel capacity. For example,a user with two receive antennas may have a different MIMO capacitycompared to a user with four receive antennas. The MIMO capacitycalculation may also take into account the effect of such factors aspath loss, shadow fading, and fast fading.

In step 904, the system 500 may identify a level of transmissionrobustness for each user based on the user's MIMO channel capacity. Forexample, a more robust transmission may be used for lower MIMO channelcapacity users so that these users can decode their signals withoutneeding to cancel out the signals of stronger users with higher MIMOchannel capacity. The stronger users with higher MIMO channel capacitymay decode the signals transmitted to weaker users with lower MIMOchannel capacity before decoding their own signals. In step 906,superposition of the user signals as previously described may occurusing the robustness for each layer identified in step 904. The signalsmay then be transmitted in step 908.

Referring to FIG. 10, in another embodiment, a method 1000 illustratesthe selection of users and corresponding code words based on the users'MIMO channel capacities in a system such as the system 500 of FIG. 5. Instep 1002, users are selected for transmission based on information 1010that represents channel information for multiple users. In step 1004, anumber of MIMO code words are determined for each user based on MIMOchannel information 1012 (e.g., channel rank). In step 1006,superposition of the user signals may occur as previously describedprior to signal transmission in step 1008.

Referring to FIG. 11, in yet another embodiment, a transmission system1100 illustrates that superimposed code words transmitted over multipletransmit antennas may be unitarily pre-coded before mapping to antennas1106 a and 1106 b. Such pre-coding may spread the transmission of eachcode word across both antennas, rather than sending the entire code wordover a single antenna. In this case, each code word may be potentiallytransmitted from two or more of the physical transmit antennas used forsuperimposed information transmission. For purposes of illustration,examples of unitary pre-coding matrices (denoted P₁ and P₂) for the caseof two transmit antennas may be described as:

${P_{1} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}},\mspace{14mu} {P_{2} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}}$

Assuming modulation symbols S1 and S2 are transmitted at a given timefrom Stream 1 and Stream 2 respectively, the modulation symbols afterpre-coding with matrix P₁ and P₂ can be written as:

$T_{1} = {{P_{1}\begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}} \times \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{S_{1} - S_{2}}\end{bmatrix}}}}$ $T_{2} = {{P_{2}\begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}} \times \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{{j\; S_{1}} - {j\; S_{2}}}\end{bmatrix}}}}$

Accordingly, the symbols

$T_{11} = {{\frac{\left( {S_{1} + S_{2}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} T_{12}} = \frac{\left( {S_{1} - S_{2}} \right)}{\sqrt{2}}}$

may respectively be transmitted from antennas 1106 a and 1106 b whenpre-coding is done using pre-coding matrix P₁.

Similarly, the symbols

$T_{21} = {{\frac{\left( {S_{1} + S_{2}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} T_{22}} = \frac{\left( {{j\; S_{1}} - {j\; S_{2}}} \right)}{\sqrt{2}}}$

may respectively be transmitted from antennas 1106 a and 1106 b whenpre-coding is done using pre-coding matrix P₂.

With additional reference to FIG. 12, inverse operations may beperformed at a receiver system 1200 using inverse matrix blocks 1202 and1204 (e.g., inv(P₁) and inv(P₂)) to recover the transmitted symbols in apre-coded MIMO system. The received symbols are multiplied with theinverse pre-coding matrices as shown below:

${{{inv}\left( P_{1} \right)} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}},\mspace{14mu} {{{inv}\left( P_{2} \right)} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & {- j} \\1 & j\end{bmatrix}}}$

It should be noted that the inverse of a unitary pre-coding matrix maybe obtained by taking the complex conjugate transpose of the pre-codingmatrix. The transmitted symbols are decoded by multiplying the receivedsymbol vector with the inverse pre-coding matrices as shown below and inFIG. 12.

${{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}} \times {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{S_{1} - S_{2}}\end{bmatrix}}} = \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}$ ${{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & {- j} \\1 & j\end{bmatrix}} \times {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{{j\; S_{1}} - {j\; S_{2}}}\end{bmatrix}}} = \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}$

Referring to FIG. 13, one embodiment of superposition coding in apre-coded MIMO system is illustrated using a system 1300 that has asuperposition coding block 1302 and a pre-coding block 1304. Thesuperposition coding performed by superposition coding block 1202 is asimple addition of the users' signals, while MIMO pre-coding isperformed in pre-coder block 1204. In this example, each user transmitstwo code words in a 2×2 MIMO system (i.e., a total of four code wordsare transmitted). For simplicity, there is only one complex modulationsymbol per code word and power gains for different users' signals havebeen omitted. However, it is understood that additional modulationsymbols may be used and, as described previously, different power gainsmay be applied to different users' signals.

Continuing the present example, modulation symbols S1, S2, S3 and S4belong to first, second, third, and fourth code words, respectively. Thefirst and second code words are transmitted to User-1, while the thirdand fourth code words are transmitted to User-2. Superposition codingblock 1302 creates a composite signal containing S1 and S3 and anothercomposite signal containing S2 and S4. The composite signals are thenfed into pre-coding block 1304, which may spread the transmission ofeach composite signal across both antennas 1306 a and 1306 b aspreviously described. Accordingly, a pre-coded composite signal

$\frac{\left( {S_{1} + S_{3} + S_{2} + S_{4}} \right)}{\sqrt{2}}$

is transmitted via antenna 1306 a and another pre-coded composite signal

$\frac{\left( {S_{1} + S_{3} - S_{2} - S_{4}} \right)}{\sqrt{2}}$

is transmitted via antenna 1306 b.

Referring to FIG. 14, one embodiment of superposition coding in apre-coded MIMO system where signals for users with different code wordsare superimposed is illustrated using a system 1400 that has asuperposition coding block 1402 and a pre-coder block 1404. In thisexample, using antennas 1406 a and 1406 b, two code words aretransmitted to User-1, while only a single code word is transmitted toUser-2.

More specifically, modulation symbols S1, S2, and S3 belong to first,second, and third code words, respectively. The first and second codewords are transmitted to User-1, while the third code word istransmitted to User-2. Superposition coding block 1402 creates acomposite signal containing S1 and S3. S2 may pass unchanged throughsuperposition coding block 1402 or may bypass the block altogether. Thecomposite signal and S2 are then fed into pre-coding block 1404, whichmay spread the transmission of both the composite signal and S2 acrossboth antennas 1406 a and 1406 b as previously described. Accordingly, apre-coded composite signal

$\frac{\left( {S_{1} + S_{3} + S_{2}} \right)}{\sqrt{2}}$

is transmitted via antenna 1406 a and another pre-coded composite signal

$\frac{\left( {S_{1} + S_{3} - S_{2}} \right)}{\sqrt{2}}$

is transmitted via antenna 1406 b.

Referring to FIG. 15, one embodiment of superposition coding in apre-coded MIMO system where different pre-coders are used for differentusers is illustrated using a system 1500 that has pre-coder blocks 1502and 1504 and superposition coding block 1506. In this example,superposition coding is performed after pre-coding. Two code words aretransmitted to User-1 using pre-coder block 1502 and two code words aretransmitted to User-2 using pre-coder block 1504.

More specifically, User-1 modulation symbols S1 and S2 are pre-codedusing pre-coder block 1502 to form

${\frac{\left( {S_{1} + S_{2}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} \frac{\left( {S_{1} - S_{2}} \right)}{\sqrt{2}}},$

and User-2 modulation symbols S3 and S4 are pre-coded using pre-coderblock 1504 to form

$\frac{\left( {S_{3} + {j\; S_{4}}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} {\frac{\left( {S_{3} - {j\; S_{4}}} \right)}{\sqrt{2}}.}$

The pre-coded information from both pre-coder block 1502 and pre-coderblock 1504 is then superimposed by superposition coding block 1506 toform composite signals

$\frac{\left( {S_{1} + S_{2} + S_{3} + {j\; S_{4}}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} \frac{\left( {S_{1} - S_{2} + S_{3} - {j\; S_{4}}} \right)}{\sqrt{2}}$

prior to transmission via antennas 1508 a and 1508 b.

Referring to FIG. 16, one embodiment of superposition coding in apre-coded MIMO system where different pre-coders are used for differentusers is illustrated using a system 1600 that has pre-coder blocks 1602and 1604 and superposition coding block 1606. In this example,superposition coding is performed after pre-coding, and two code wordsare transmitted to User-1 using pre-coder block 1602 while only asingle-code word is transmitted to User-2 using pre-coder block 1604.

More specifically, User-1 modulation symbols S1 and S2 are pre-codedusing pre-coder block 1602 to form

${\frac{\left( {S_{1} + S_{2}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} \frac{\left( {S_{1} - S_{2}} \right)}{\sqrt{2}}},$

and User-2 modulation symbol S3 is pre-coded using pre-coder block 1604to form

$\frac{S_{3}}{\sqrt{2}}\mspace{20mu} {and}\; {\frac{S_{3}}{\sqrt{2}}.}$

The pre-coded information from both pre-coder block 1602 and pre-coderblock 1604 is then superimposed by superposition coding block 1606 toform composite signals

$\frac{\left( {S_{1} + S_{2} + S_{3}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} \frac{\left( {S_{1} - S_{2} + S_{3}} \right)}{\sqrt{2}}$

prior to transmission via antennas 1608 a and 1608 b.

Referring to FIG. 17, in yet another embodiment, a method 1700illustrates the superposition of user signals based on each user'schannel rank (e.g., the rank of the channel matrix H) in a system suchas the system 500 of FIG. 5. In step 1702, users are sorted by channelrank based on MIMO information 1710 (e.g., channel rank information).

In step 1704, a level of transmission robustness to be used with eachuser may be identified based on rank. For example, a higher level ofrobustness may be identified for lower ranked users, while a lower levelof robustness may be identified for higher ranked users.

In step 1706, user signals may be superimposed prior to transmission instep 1708. For example, the signals may be superimposed so that lowerrank signals are decoded with higher reliability. This may be achievedby using a higher power gain, beamforming, or a more robustmodulation/coding scheme for the lower rank signals. This approach mayenable higher rank users to decode lower rank signals and cancel thesesignals effectively. Furthermore, as lower rank users may noteffectively decode the higher rank signals, lower rank signals should beable to be decoded without the need for decoding and canceling thehigher rank signals.

With additional reference to FIG. 18, the sorting that occurs by channelrank (e.g., in step 1702 of FIG. 17) may result in multiple users havingthe same channel rank. Accordingly, in addition to or as an alternativeto step 1702 of FIG. 17, the condition number of the matrix H (whichrepresents the spread in singular values of the matrix) may be used insorting the users. In general, a lower spread represents a highercapacity channel.

Accordingly, in step 1802, users are sorted by channel condition numberbased on MIMO channel information 1810. In step 1804, a level oftransmission robustness to be used with each user may be identifiedbased on the channel condition number. For example, a higher level ofrobustness may be identified for users associated with a higher channelcondition number, while a lower level of robustness may be identifiedfor users associated with a lower channel condition number. In step1806, user signals may be superimposed in such a way that signalscorresponding to users having a lower channel condition number aredecoded first. Transmission may then occur in step 1808.

Referring to FIGS. 19 a and 19 b, in still another embodiment, methods1900 and 1910 illustrate user signal decoding in a situation wheresuperposition coding is based on user MIMO channel rank (i.e., asillustrated with respect to FIG. 17). Method 1900 of FIG. 19 aillustrates decoding by a User-1, who has a higher channel rank thanUser-2 (whose decoding is represented by method 1910 of FIG. 19 b). Forpurposes of illustration, the decoding in the present examplecorresponds to User-1 and User-2 signal superposition coding as shown inFIG. 16. More specifically, referring also to FIG. 16, User-1 transmitstwo code words on a rank 2 MIMO channel, while User-2 transmits a singlecode word on a rank 1 MIMO channel.

As illustrated in FIG. 19 a, User-1 with MIMO channel rank 2 firstdecodes user-2 signal S3 (FIG. 16) in step 1902 and cancels it from thereceived signal in step 1904. In step 1906, User-1 then decodes its ownsignals S1 and S2. It is noted that User-1 with a rank 2 MIMO channel iscapable of detecting and decoding the rank 1 transmission for User-2.However, User-2 and its corresponding rank 1 MIMO channel may not beable to decode User-1 signals transmitted on the rank 2 MIMO channel.Accordingly, as illustrated in step 1912 of FIG. 19 b, User-2 may decodeits signal S3 without decoding and canceling User-1 signals S1 and S2.

Referring to FIG. 20, in another embodiment, a system 2000 illustrateshow a broadcast service may be superimposed on a unicast MIMO service.Generally, a broadcast is a point-to-multipoint service that isdecodable by a majority of the users in a network. Accordingly, in thepresent embodiment, the broadcast stream may be detected and cancelledbefore unicast decoding occurs to improve the overall system efficiency.

In the present example, the system 2000 includes a single-streamtransmission for the broadcast traffic and a two-stream MIMOtransmission for the unicast traffic. The system 2000 includes Turboblocks 2002 a-2002 c and modulation blocks 2004 a-2004 c that handle thefirst unicast transmission stream (CW1), the broadcast transmissionstream, and the second unicast transmission stream (CW2), respectively.All or a portion of the broadcast transmission stream may then becombined with the first unicast transmission stream at block 2006 abefore entering IFFT block 2008 a. The IFFT blocks 2008 a and 2008 bfeed into a pre-coding block 2010, which spreads the signals (includingthe broadcast signal) over antennas 2012 a and 2012 b for transmission.

Referring to FIG. 21, in yet another embodiment, a system 2100illustrates how a broadcast service may be superimposed on a unicastMIMO service. In the present example, the system 2100 includes twostreams for the broadcast traffic and a two stream MIMO transmission forthe unicast traffic. The system 2100 includes Turbo blocks 2102 a-2102 dand modulation blocks 2104 a-2104 d that handle the first unicasttransmission stream (CW1), the first broadcast transmission stream(CW1), the second unicast transmission stream (CW2), and the secondbroadcast transmission stream (CW2), respectively.

In the present example, the first broadcast transmission stream may becombined with the first unicast transmission stream at block 2106 aafter coding and modulation, and the combined signal may be passedthrough IFFT block 2108 a. Similarly, the second broadcast transmissionstream may be combined with the second unicast transmission stream atblock 2006 b after coding and modulation, and the combined signal may bepassed through IFFT block 2108 b. The IFFT blocks 2108 a and 2108 b feedinto a pre-coding block 2110, which spreads the signals (including thebroadcast signal) over antennas 2112 a and 2112 b for transmission.

Referring to FIG. 22, in another embodiment, a decoding process 2200 isillustrated that may be used by a receiver to decode superimposed layersof a two-stream transmission for both unicast and broadcast, such asthose encoded by the system 2100 of FIG. 21. It is noted that a total ofthree interference cancellation operations are performed in this case.In a situation involving a single-stream broadcast transmission and a2-stream unicast transmission (e.g., FIG. 20), two cancellationoperations are performed. In a situation involving a single-streamtransmission for both unicast and broadcast, a single interferencecancellation operation is performed.

In the present example, beginning with the first broadcast stream, anMMSE operation is performed in block 2202 and the stream is decoded inblock 2204. The first broadcast stream is cancelled from the receivedsignal in block 2206, and the resulting signal is fed into block 2208.In block 2208, an MMSE operation is performed and the second broadcaststream is decoded in block 2210. The first and second broadcast streamsare cancelled from the received signal in block 2212. If the receiver isa broadcast user, both streams of the broadcast signal have been decodedand processing may stop at this point. Otherwise, the resulting signalfrom block 2212 is fed into the next block

If the receiver is a unicast user, processing continues after cancelingthe first and second broadcast streams in block 2212. In block 2214, anMMSE operation is performed and the first unicast stream is decoded inblock 2216. The first and second broadcast streams and the first unicaststream are cancelled from the received signal in block 2218, and theresulting signal is fed into block 2220. In block 2220, an MRC operationis performed and the second unicast stream is decoded in block 2122.

Accordingly, additional processing may be needed to decode bothbroadcast and unicast traffic when the signals are superpositioned overa MIMO system as described. However, an amount of buffering needed forcancellation may be substantially the same as is needed in a MIMOinterference cancellation receiver, even when multiple interferencecancellation operations are performed.

Referring to FIG. 23, in yet another embodiment, a system 2300illustrates how a multi-stream broadcast service may be pre-coded beforebeing transmitted. In the present example, the system 2300 sends twobroadcast traffic codewords S1 and S2 through pre-coding block 2302.More specifically, broadcast modulation symbols S1 and S2 are pre-codedusing pre-coder block 2302 to form

${\frac{\left( {S_{1} + S_{2}} \right)}{\sqrt{2}}\mspace{14mu} {and}\mspace{14mu} \frac{\left( {S_{1} - S_{2}} \right)}{\sqrt{2}}},$

which are then sent via antennas 2304 and 2306. As previously described,the broadcast traffic may be superimposed with other traffic before orafter the pre-coding occurs.

Referring to FIG. 24, in still another embodiment, a system 2400illustrates how a single-stream broadcast service may be pre-codedbefore being transmitted. In the present example, the system 2400 sendsa single broadcast traffic codeword S1 through pre-coding block 2402.More specifically, broadcast modulation symbol S1 is pre-coded usingpre-coder block 2302 to form

${\frac{S_{3}}{\sqrt{2}}\mspace{20mu} {and}\; \frac{S_{3}}{\sqrt{2}}},$

which are then sent via antennas 2404 and 2406. As previously described,the broadcast traffic may be superimposed with other traffic before orafter the pre-coding occurs.

Referring to FIG. 25, in another embodiment, a method 2500 illustrates aprocess by which pre-coding of a broadcast signal may occur within aMIMO system, such as the system 2400 of FIG. 24. In step 2502, thebroadcast signal is modulated to create a stream of modulated symbols.In step 2504, pre-coding is performed on each of the modulated symbolsto distribute each modulated symbol across multiple antennas (e.g., theantennas 2404 and 2406 of FIG. 24). In step 2506, the pre-coded symbolsmay be transmitted.

Although only a few exemplary embodiments of this disclosure have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this disclosure. For example, various features describedherein may be implemented in hardware, software, or a combinationthereof. Also, features illustrated and discussed above with respect tosome embodiments can be combined with features illustrated and discussedabove with respect to other embodiments. For example, various steps fromdifferent flow charts may be combined, performed in an order differentfrom the order shown, or further separated into additional steps.Furthermore, steps may be performed by network elements other than thosedisclosed. Accordingly, all such modifications are intended to beincluded within the scope of this disclosure.

1. A method comprising: demultiplexing a first signal of a first userinto at least first and second signal portions; demultiplexing a secondsignal of a second user into at least third and fourth signal portions;performing superposition coding on the first and third signal portionsto form a first composite signal and performing superposition coding onthe second and fourth signal portions to form a second composite signal;and transmitting the first composite signal via at least a first antennaof a multiple input multiple output (MIMO) system and transmitting thesecond composite signal via at least a second antenna of the MIMOsystem.
 2. The method of claim 1 wherein performing superposition codingon the first, second, third, and fourth signal portions results in thefirst and second signal portions being in a first superposition layer ofthe first and second composite signals and the third and fourth signalportions being in a second superposition layer of the first and secondcomposite signals.
 3. The method of claim 1 further comprising applyinga first gain to at least one of the first and second signal portions andapplying a second gain to at least one of the third and fourth signalportions.
 4. The method of claim 3 wherein the gain is applied after thedemultiplexing and before the superposition coding.
 5. The method ofclaim 1 further comprising sorting the first and second users based onMIMO channel information associated with the first and second users toidentify a transmission scheme to be used with each of the first andsecond users.
 6. The method of claim 5 wherein the sorting is based on aMIMO channel rank of each of the first and second users.
 7. The methodof claim 5 wherein the sorting is based on a MIMO channel conditionnumber of each of the first and second users.
 8. The method of claim 1further comprising performing pre-coding on the first and secondcomposite signals to form first and second pre-coded signals that eachcontain a portion of the first and second composite signals, wherein thefirst and second pre-coded signals are transmitted via the first andsecond antennas, respectively.
 9. A method comprising: demultiplexing afirst signal of a first user into at least first and second signalportions; demultiplexing a second signal of a second user into at leastthird and fourth signal portions; performing pre-coding on the first andsecond signal portions to form first and second pre-coded signals thateach contain a portion of the first and second signal portions;performing pre-coding on the third and fourth signal portions to formthird and fourth pre-coded signals that each contain a portion of thethird and fourth signal portions; performing superposition coding on thefirst and third pre-coded signals to form a first composite signal andperforming superposition coding on the second and fourth pre-codedsignals portions to form a second composite signal; and transmitting thefirst composite signal via at least a first antenna of a multiple inputmultiple output (MIMO) system and transmitting the second compositesignal via at least a second antenna of the MIMO system.
 10. The methodof claim 9 further comprising sorting the first and second users basedon MIMO channel information to identify a transmission scheme to be usedwith each of the first and second users.
 11. The method of claim 10wherein the sorting is based on a MIMO channel rank of each of the firstand second users.
 12. The method of claim 10 wherein the sorting isbased on a MIMO channel condition number of each of the first and secondusers.
 13. A method comprising: performing superposition coding on afirst signal of a first user and a second signal of a second user tocreate a composite signal; demultiplexing the composite signal intofirst and second composite signal portions, wherein each of the firstand second composite signal portions contains a portion of the firstsignal and the second signal; and transmitting the first compositesignal portion via a first antenna of a multiple input multiple output(MIMO) system and transmitting the second composite signal portion via asecond antenna of the MIMO system.
 14. The method of claim 13 furthercomprising applying a first gain to the first signal and a second gainto the second signal.
 15. The method of claim 13 further comprisingsorting MIMO channel information of the first and second users, whereinthe sorting is used to assign each of the first and second signals to asuperposition layer in the first and second composite signals.
 16. Themethod of claim 13 further comprising performing pre-coding on the firstand second composite signals to form first and second pre-coded signalsthat each contain a portion of the first and second composite signals,wherein the first and second pre-coded signals are transmitted via thefirst and second antennas, respectively.
 17. A method comprising:receiving, by a receiver, at least first and second associated multipleinput multiple output (MIMO) signals, wherein each of the first andsecond signals contains at least corresponding first and secondsuperimposed layers; decoding the first layer of the first and secondsignals; and decoding the second layer of the first and second signalsafter decoding the first layer.
 18. The method of claim 17 wherein thesecond layer is decoded only if the second layer contains informationassociated with the receiver.
 19. The method of claim 17 whereindecoding the first layer includes: decoding a first code word associatedwith the first layer; combining the first code word with at least one ofthe first and second signals to cancel the first code word from the atleast one first and second signal; and decoding a second code wordassociated with the first layer.
 20. The method of claim 17 wherein thefirst layer contains broadcast information and wherein the second layercontains unicast information.
 21. A method comprising: demultiplexing aunicast signal into at least first and second unicast signal portions;performing superposition coding on the first unicast signal portion anda broadcast signal to form a first composite signal and performingsuperposition coding on the second unicast signal portion and thebroadcast signal to form a second composite signal; and transmitting thefirst composite signal via at least a first antenna of a multiple inputmultiple output (MIMO) system and transmitting the second compositesignal via at least a second antenna of the MIMO system.
 22. The methodof claim 21 further comprising demultiplexing the broadcast signal intoat least first and second broadcast signal portions, wherein performingthe superposition coding to form the first composite signal includessuperposition coding the first unicast signal portion and the firstbroadcast signal portion and wherein performing the superposition codingto form the second composite signal includes superposition coding thesecond unicast signal portion and the second broadcast signal portion.23. The method of claim 21 wherein the unicast signal is multi-streamand the broadcast signal is single stream.
 24. The method of claim 21wherein both the unicast and broadcast signals are multi-stream.
 25. Themethod of claim 21 further comprising pre-coding the first and secondcomposite signals, where a portion of the first and second compositesignals is sent over each of the first and second antennas.
 26. Themethod of claim 21 further comprising pre-coding at least one of theunicast signal and the broadcast signal prior to performing thesuperposition coding.
 27. A multiple input multiple output (MIMO) systemcomprising: a modulation block configured to modulate a first signalinto at least first and second modulated symbols and to modulate asecond signal into at least third and fourth modulated symbols; a serialto parallel conversion block configured to parallelize the first andsecond modulated symbols and the third and fourth modulated symbols; asuperposition coding block configured to superimpose the first and thirdmodulated symbols to form a first composite signal and to superimposethe second and fourth modulated symbols to form a second compositesignal; and first and second antennas coupled to the superpositioncoding block and configured to transmit at least the first and secondcomposite signals, respectively.
 28. The system of claim 27 furthercomprising a parallel to serial conversion block to serialize the firstand second composite signals prior to transmission.
 29. The system ofclaim 27 further comprising a pre-coding block coupled to thesuperposition coding block and configured to distribute each of thefirst and second composite signals across each of the first and secondantennas for transmission.
 30. The system of claim 27 further comprisinga pre-coding block coupled to the demultiplexing block and configured todistribute each of the first, second, third, and fourth modulatedsymbols across each of the first and second antennas for transmission.